1. Field of the Invention
Embodiments of the present invention relate to a technology to reduce power consumption in switching regulators, and, in particular, to a technology to reduce power consumption in voltage detection circuits that use photo-couplers.
2. Description of the Related Art
FIG. 8 shows an example of the circuit of a current mode quasi-resonant type switching regulator of a conventional technology disclosed in Japanese Unexamined Patent Application Publication No. 2007-215316 (also referred to herein as “Patent Document 1”). The switching regulator includes a transformer T1 that has a primary winding WP1, secondary winding WS1, and an auxiliary winding WP2. An end of the primary winding WP1 is connected to an input terminal Pi and the other terminal is connected to the drain of the MOSFET of a switching element Q1. An end of the secondary winding WS1 is connected to a positive output terminal Po through a diode D1, and the other end is connected to a negative output terminal No. An end of the auxiliary winding WP2 is connected to a ZCD terminal that is an input terminal for zero current detection in a switching control circuit IC1, and the other end is connected to the ground GND of the primary side circuit.
A smoothing capacitor Ci is connected between the positive input terminal Pi and the negative input terminal Ni; a smoothing capacitor Co is connected between the positive output terminal Po and the negative output terminal No; and a resonance capacitor Cr is connected between the drain and source of the switching element Q1 in parallel with the switching element Q1. Between the positive output terminal Po and the negative output terminal No also connected is a voltage-dividing circuit consisting of resistors Ro1 and Ro2. A resistor Rs for current detection is connected between the source of the switching element Q1 and the ground.
The switching control circuit IC1 contains: a bottom detecting circuit BD, a switching width bottom controlling circuit SWC, an OR circuit G2, a one-shot circuit OS, a restart circuit RST, a flip-flop FF, a driving circuit DRV, a comparator CP, a burst circuit BC, and an AND circuit G1. This switching control circuit IC1 is composed as a semiconductor integrated circuit. The terminal OUT of the switching control circuit IC1 is connected to the gate of the switching element Q1; the terminal SI, to the connection point between the source of the switching element Q1 and the resistor Rs; and the FB terminal, to the collector of a photo-transistor of a photo-coupler PC1 for output voltage detection. Since Patent Document 1 describes the circuit construction and operation of the light emitting diode side of the photo-coupler PC1, the description thereon is omitted here.
The bottom-detecting circuit BD detects a bottom (or a locally minimum state) of the output voltage of the auxiliary winding WP2 applied to the ZCD terminal, and delivers a bot signal, which is fed to the switching width bottom control circuit SWC.
The switching width bottom control circuit SWC determines whether to output a bot_out signal based on the first bot signal or to output a bot_out signal based on a later bot signal. The one-shot circuit OS outputs a set signal, which sets the flip-flop FF. Consequently, the output Q of the flip-flop FF becomes an H level signal of driving signal dry to turn ON the switching element Q1. When the switching element Q1 turns ON, the current increases and is detected with the resistor Rs as a voltage. When this voltage reaches the voltage at the terminal FB to which the collector of the photo-transistor of the photo-coupler PC1 for detecting the voltage between the output terminals Po and No is connected, the output of the comparator CP becomes an H signal to reset the flip-flop FF. As a consequence, the switching element Q1 turns OFF and the magnetic energy stored in the transformer T1 during the ON state is transferred to the secondary winding WS1 and through the diode D1, to the DC output capacitor Co charging the capacitor Co and raising the output voltage. By repeating the processes, the DC output voltage is controlled at a predetermined constant voltage.
The burst circuit BC is provided to decrease the number of switching during no loading or light loading state and reduce power consumption. When the output signal bur from the burst circuit BC is L (at a low level), the AND circuit G1 interrupts the driving signal drv.
The resistor R1 connected between the reference voltage Vref in the control circuit IC1 and the terminal FB works for converting the feedback signal from the photo-coupler to a voltage. When the DC output voltage is going to decrease, the photo-coupler works to raise the voltage at the terminal FB, and when the DC output voltage is going to increase, the photo-coupler works to lower the voltage at the terminal FB. These operation controls the current through the switching element Q1 to control the DC output voltage at a predetermined constant voltage.
In the above-described example of switching regulator, a photo-coupler is used in the feedback circuit to detect the DC output voltage in the secondary side of the transformer with electrical insulation. The current Icur through the photo-transistor connected in the primary side circuit is determined from the voltage Vref applied to the pull-up resistor R1, a resistance value Ra of the pull-up resistor R1, and a control voltage Vfb, which is a voltage at the terminal FB, and given by the following equation (1).Icur=(Vref−Vfb)/Ra  (1)
The current If through a light-emitting diode connected to the secondary side circuit of the photo-coupler is given by the following equation (2).If=Icur/CTR=(Vref−Vfb)/Ra/CTR  (2),
where CTR is a current transfer ratio of the photo-coupler.
Equations (1) and (2) indicate that a heavy load condition (with large power consumption in the load), at which the voltage Vfb at the terminal FB is high, results in a small current Icur and If; a light load condition (with small power consumption in the load), at which the voltage Vfb at the terminal FB is low, results in a large current Icur and If. Therefore, power load due to the current Icur and current If during a low load period and no load period is larger than that during a heavy load period. The power consumption during a waiting period, in particular, is a problem to be solved.
To reduce the power consumption during a waiting period, it is an effective means to decrease the current Icur and If. Although a larger pull up resistance value could decrease the current Icur and current If, the pull up resistance value must be set at such a value that the current Icur at the maximum power is larger than the dark current, which is a current flowing in the photo-transistor under a state of no light emission from the light emitting diode, of the photo-coupler. Otherwise, the DC output voltage cannot be detected to fail to supply the maximum power.
FIG. 9 shows a relationship between the current Icur through the photo-transistor and the control voltage Vfb. The Icur at the control voltage Vfb_0 at the maximum load must be selected at a value larger than the dark current Idark of the photo-coupler. As shown in FIG. 9, this condition is satisfied in the case of a small R1 value, while the condition is not satisfied in the case of a large R1 value.